Back when I was an exploration geologist looking for uranium, one of the things we would do was wander around prospecting with a scintillometer. A typical background on sandstone would be ~40 cps, whereas we could expect >400 cps on granite, an order of magnitude variation.

What exactly did the article define as the background? The ambient conditions of the laboratory?

Also, it is not just the uranium in the granite that is responsible for the radiation. Thorium and potassium are significant as well.